The Role of Traffic Volume in Habitat Connectivity and Mortality

1. The Role of Traffic Volume in Habitat Connectivity and Mortality Sandra Jacobson June 2007

2. Most animal species make movements of at least 4 types • Home range (regular movements within a fixed area) • Migratory (seasonal movements between breeding and non-breeding habitats) • Dispersal (movements by juveniles away from the natal area) • Geographic range shifts (long-term movements in response to climatic or environmental changes) • From Hunter 1997

3. Natal dispersal movements • Increasing evidence that this movement type is critical to maintain genetic and demographic exchange among established populations (see de Maynardier and Hunter 2000 p 62) • Roads of all types filter demographic groups differently • Dispersing and migrating small mammals cross roads more than resident ones (Swihart and Slade 1984)

4. Traffic Volume as a Predictor of Impacts • Volume is collected consistently with national standards • DOTs understand it • Good conceptual as well as empirical basis • More research needed to verify species-specific effects

5. Definition of Traffic Volume • Volume=Speed x Density • Volume=vehicles/time • Speed=distance/time • Density=vehicles/distance • VOLUME (Vehicles/time)= Speed (distance/time) x Density (vehicles/distance)

6. At LOW traffic intensity the small proportion of fauna casualties and animals repelled causes limited impact on the proportion of animals successfully crossing a barrier. • At MEDIUM traffic intensity casualties are high, the number of animals repelled by the infrastructure increases and the proportion of successful crossings decreases. • At HIGH traffic intensity a large proportion of animals are repelled and despite a lower proportion of fauna casualties there is only a small proportion of successful crossings. (Graph redrawn from Andreas Seiler, unpublished, text from COST-341 Handbook)

7. Two Major Types of Response to Traffic Volume • Large swift animals • usually smart and exhibit avoidance behavior • >>Barrier effect predominates • Small slow animals • often not smart and move regardless of intimidation • Easy to mathematically predict complete barrier • >>Mortality effect predominates

8. Implications for Interpretation of Research Results • Impacts do not increase linearly • Traffic volumes must be identified by appropriate measures • Vehicles/hr for the activity period of the research species • Past research has been sloppy on terms and categories of volume • Volume measurements need to be accurate and precise for road in question • Define ‘high’ volume

9. Movement Groups • Mathematically, traffic volume is straightforward: • Speed of animals vs gap in traffic • Movement groups allow for an organizing principle • Can be used across regions

10. Research on the Relationship of Traffic Volume to Impacts • Van Langevelde and Jaarsma • Hels and Buchwald • Aresco • Waller • Dodd and Gagnon

11. Probability of Successful Crossing: Parameters (Van Langevelde and Jaarsma 2004) • Traffic volume • Traversing speed of animals • Body length (size) • Road width • Larger spp that travel rapidly are the least vulnerable to roadkill per crossing attempt (compared to other combinations) • Because usually there are fewer large animals than smaller ones, large animal populations may be affected by absolute number losses compared to smaller animals • Road crossings during daily movements as compared with fewer dispersal movements increase prob of mortality because of increased exposure to risk

12. Probability of successful crossing (van Langevelde and Jaarsma 04) • Traffic volume and traversing speed have largest effect on probability of success • Increase in volume dramatically reduces P for slow animals regardless of body size • Model can be used to compare changes in conditions • Effectiveness monitoring of mitigation measures • Predicted future impacts from changed conditions such as projected volume increases • See van Langevelde and Jaarsma 1997 (proceedings) for example

13. Determination of Probability of successful road crossing • Sensitivity analysis: Jaarsma 04 • Exact determination of pavement width, traversing speed of animal, traffic volume is needed when: • animals are slow or… • traffic volume is high • Exact determination of traversing speed and body length is NOT necessary for larger mammals for roads with lower volume or pavement width

14. Number of Roadkills Van Langevelde and Jaarsma 2004 • Number of roadkills in a given time period estimated by • D=(1-P)K • D= number of roadkills • P= probability of successfully crossing once • K=number of crossing attempts

15. Hels and Buchwald 2001: Probability of small animals crossing road alive • p = e-Na/v • Where p is the probability of a successful crossing of one individual for one crossing attempt; N is the traffic volume per unit time (in AADT, Average Annual Daily Traffic); a is the combined kill zone of the animal size and vehicle surface; and v is the velocity of the animal (Hels and Buchwald 2001). • This simplified equation assumes a perpendicular crossing thus it overestimates successful crossings. • Jaeger and Fahrig (2004) determined that when the probability of successful crossing fell below 0.2, population persistence increased in all cases where access to a highway was restricted; therefore this value seems reasonable to define a complete barrier.

16. Differences between H&B and VL&J Probability Approaches • VL&J • Uses traffic flow theory • consider entire paved road width to be effective killing surface (approximately true for large animals) • assumes traffic operates on a Poisson distribution which limits range of use to ca. 5000 AADT • Does not consider crossing angle • H&B • Consider killing width based on tire size and animal size because some animals can be on the road surface with a passing vehicle and survive • Range of applicable AADT’s not stated but model is limited to same traffic flow theory range • Can account for crossing angle where known

17. Waller et al 2005 Probability of successfully crossing road • Also uses traffic flow theory • Couched in more difficult to use format but the idea is the same: the longer an animal is on the road, the more risk it incurs

18. Dodd and Gagnon in Arizona • Ongoing research • Radiotelemetered elk • Highest collision rate is on weekdays; highest traffic is on weekends • Difference in traffic volume is only about 3000 vehicles per day (i.e. 5000-8000) • Animals travel farther to cross at high traffic volume times

19. Traffic Calming (1) • Definition: adaptation of roads to reduce vehicle conflicts with other road users • Traffic calming can enhance permeability by rearranging traffic so that volume on important roads is reduced • Concept is important because volume is biggest impact, so volume can be managed for more favorable conditions for wildlife • Can rearrange traffic so that people can access needed areas but all roads are not increased to high levels of service

20. Traffic Calming (2) • ‘Rat Running’ is through traffic using lower level of service roads to avoid higher traffic areas and to save time or distance (Jaarsma 97) • Avoid ‘rat running’ by education and maintaining level of service at lower level • See Jaarsma in Canters • See Jaarsma and Willems 2002 • ***This is a really good reason to mitigate very low volume roads, because then when traffic is on low volume roads they are essentially non-barriers or mortality risks

21. Ausbau vor Neubau • Building up existing roads (Ausbau) is less impactive to wildlife than building new roads (Neubau) • See Jaeger and Fahrig (2004)

22. Selected Online Resources • Interactive online model for road effects http://www.nls.ethz.ch/roadmodel/index.htm • Least cost pathway algorithm example http://www.grizzlybear.org/leastcostpath.htm • http://www.biodiversitypartners.org/ • Conservation design section: http://www.biodiversitypartners.org/habconser/cnd/index.shtml • Graphics for conservation design principles: http://www.biodiversitypartners.org/habconser/cnd/principles.shtml